Recycle backend connection before setting BOS_FINISHED (take two)

This to give predictable backend reuse behavior for varnishtest,
making the test cases non-racy.

2nd version of this patch. This version will only deal with recyclable
backend connections before BOS_FINISHED, leaving the slower close path
to the clean up code that is run after the BOS_FINISHED state.

Fixes: #1491
Be consistent with when the BerespBody timestamp is taken.
Add two counters, one counts purge operations, the other objects purged.

Suggested by:	Steven Engelhardt via github
Remove extra whitespace.
Add support for setting coresize from sysconfig.

Add support for using this if it is defined by the administrator
in the /etc/sysconfig/varnish file.

From way back we've had a commented-out DAEMON_COREFILE_LIMIT in
our /etc/sysconfig/varnish file, which wasn't referenced anywhere.

Suggested by: gaoyongwei/itxx00 on github.
Up the repeat count for slow test-enviroments
Update the grace docs to 4.0
Syntax fixed